2018 tailings and mine waste onference lternative

| Geotechnical Engineering Centre1

2018 TAILINGS AND MINE WASTE CONFERENCE

ALTERNATIVE APPROACHES TO MANAGEMENT

AND CLOSURE OF TAILINGS STORAGE

FACILITIES

Professor David Williams

Director Geotechnical Engineering Centre

The University of Queensland, Brisbane, Australia

Email: [email protected]

CRICOS Provider No 00025B


Scope

• Extending Net Present Value approach to tailings

management and closure

• Conventional approach to tailings disposal and storage

• Tailings dam failure rates

• Risk of tailings liquefaction or flow

• Tailings closure and rehabilitation risks and costs

• Alternative approaches to tailings disposal, storage and

rehabilitation



Net Present Value Approach

• NPV, with a high Discount Factor (typically 6 to 10%,

several times inflation rate) is applied to achieve financing

of high cost mining projects

• NPV is extended to tailings disposal, minimising short-

term capital costs, with rehabilitation costs discounted by

same high Discount Factor, which contributes to:

– Increased risk of tailings dam failure

– Initially inexpensive but too small surface facilities storing tailings

slurry delivered by robust and inexpensive centrifugal pumps

– Wet and soft tailings deposits, excessive stored water, and an

ever-increasing stored volume and footprint.

– A blow out in costs, as frequent tailings dam raises are required,

and unintended and cumulative negative impacts

– Difficult and costly rehabilitation, at a time when revenues cease



Constraints Under Which a Surface

TSF Must Operate

• Climatic and topographic setting of TSF

• Dam foundation conditions, and availability and suitability

of borrow materials

• Nature of tailings, particularly presence of clay minerals

• Tailings production rate that must be stored and % solids

at which they are deposited

• Need to manage, store and recycle supernatant

• Need to meet discharge water quality licence conditions

• Need to maximise tailings settled dry density, and hence

minimise wall raising and volume of tailings stored

• Desirability of upstream raising, where appropriate

• Need to rehabilitate TSF on closure



Conventional Approach to Tailings

Disposal and Storage

• Disposal and storage of tailings slurry

• Tailings containment and method of construction and dam

raising varies from region to region:

– Upstream construction is widely employed in South Africa,

Australia and south-west of USA, which share a dry climate

– Downstream construction is employed in wet regions, such as

Canada

– Sand dams, cycloned and/or compacted, are widely employed in

South America, usually raised by centreline method

– Roller compacted concrete dams are employed for high tailings

dams in deep valleys of Andes in South America

• While downstream construction is understandable in wet

climates, choice between upstream construction and sand

dams is not so obvious – “What we’ve always done”



Tailings Continuum (adapted from

Davies and Rice, 2004)


By gravity – U/G or In-Pit


Conventional Approach to Tailings

Disposal and Storage

• A commonly held perception, supported by NPV

approach, is that transporting tailings as a slurry to a

surface dam is most economic

• Discounted cost of rehabilitating resulting TSF is not

considered to be significant

• However, few TSFs have been rehabilitated, due to

difficulty and expense of capping a “slurry-like” (wet and

soft) tailings deposit

• Filtering tailings is perceived to be too expensive due to

capital and operating costs of plant and transport

• Filtered tailings occupy less volume, are relatively easy to

cap, and may be rehabilitated to a high level of future land

use and/or ecological function



Ongoing Tailings Dam Failures

• Average tailings dam failure rate over last 100 years is

1.2% or 2.2/year, >2 orders of magnitude higher than that

for water retention dams of 0.01%

• USA has reported highest number of tailings dam failures

of any country, with 39%,18% located in Europe, 12% in

Chile, and 5% in Philippines

• Most tailings dams that failed were of moderate height – 5

to 20 m high

• Focus is on failures that occur in developed countries

(e.g., Mount Polley in 2014, Cadia in 2018) or that involve

global mining companies (e.g., Samarco in 2015)

• Recent, high profile tailings dam failures are threatening

mining industry’s financial and “social licence to operate”!



Responses to Recent High Profile

Tailings Dam Failures

• Reviews of existing tailings dams by major mining

companies, or when required by Regulators

• More conservative designs, including using lowest bound

parameters – Questioning safety of previous designs, or

feasibility of failure through only lowest strength materials!

• Banning some tailings containment practices, such as

upstream construction in Chile’s seismic environment

• Increased liability of “Engineer of Record”/Design Engineer

• Concern about personal liability/prosecution of senior

executives, driving tailings management

• Increased Security Bonds and Guarantees for TSFs



Responses to Recent High Profile

Tailings Dam Failures

• Responses have not (yet) included a major shift in existing

tailings slurry disposal and containment

• What has occurred is:

– Increased tailings dam monitoring (in real-time), particularly at

failure sites

– Inclusion of buttresses as an “insurance policy”, sometimes forced

by inadequate tailings management and monitoring

– Conditions being imposed on tailings management at new mines

– Filtering of tailings being considered beyond desert sites, possibly

including co-disposal with waste rock



570 mm January 2016 Rainfall-

Induced Tailings Run-Out



Risk of Tailings Liquefaction

• Conditions for liquefaction of tailings:

– Silty sand to fine-grained sandy tailings

– Loose (contractive, brittle) state

– Near-saturated

– Trigger event:

• Earthquake magnitude > 5.5 and peak ground acceleration >

0.13g

• For “static” liquefaction:

– Too rapid a rate of upstream construction

– Too rapid a rate of rise of tailings

– A flooding event, possibly leading to overtopping and

erosion of embankment

• Susceptible tailings can behave in an undrained,

contractive, strain-softening manner, and liquefy or flowCRICOS Provider No 00025B


Risk of Tailings Liquefaction

• Davies et al. (2002) claimed that tailings impoundments

have demonstrated more static liquefaction events than

any other cause

• Risk of static liquefaction may be higher at sites with low

design earthquakes, such as in Australia

• Tailings potentially susceptible to liquefaction are difficult

to sample and assess

• A precautionary approach is to assume that identified

“loose” tailings will be susceptible to static liquefaction,

and to outlaw upstream construction

• However, well-managed upstream construction can safely

support this method



Tailings Operation, Closure and

Rehabilitation Costs

• Use of NPV and an artificially high Discount Factor result

in:

– Apparent cost savings in tailings management in short-term

– Increasing operational and capital costs over time

– Unintended cumulative detrimental impacts over time

– Ever-increasing closure and rehabilitation risks and costs in long-

term

• To illustrate this, simple NPV analyses are applied to

alternative management and closure approaches for:

– Coal tailings from open pit mining operations

– In relatively flat terrain of Eastern Australian Coalfields

– Mine life of 20 years

– AUD75,000/ha for rehabilitation



Relative Comparison of Tailings

Storage Alternatives


High cost

of Dams in

flat terrain

High cost

of RehabHigh cost of

re-handlingHigh CapEx

Balancing

Dam &

Rehab costs


10% Discount Factor Applied to

Tailings Operations & Rehabilitation


A high Discount

Factor makes

(delayed)

rehabilitation

“appear cheap”,

with rehabilitation at

closure “cheapest”,

but this ignores

cumulative

unintended

impacts, Regulators

now favouring

“Progressive

Rehabilitation”, and

ongoing Security

Deposit payments


2.5% Discount Factor Applied to

Tailings Operations & Rehabilitation


A more realistic

Discount Factor of

2.5% indicates

more realistic,

higher

rehabilitation costs


Tailings Security Deposit

• Queensland and NSW indicative costs for reshaping,

capping/sealing tailings are:

– $170,000/ha for tailings likely to present considerable difficulties

due to reactive and/or soft tailings.

– $108,000/ha for tailings likely to present moderate difficulties due

to reactive and/or soft tailings.

– $81,000/ha for benign and strong tailings.

• To these are added costs for land preparation and

revegetation, and maintenance, giving a total Security

Deposit from $85,300 to $215,000/ha

• Actual cost could be far less, under favourable conditions

• A Bank Guarantee would cost ~1.5 to 3% pa of a high

Security Deposit, possibly continuing in perpetuity



Alternative Approaches to Tailings

Disposal, Storage and Rehabilitation

• Rather than focussing on lowest NPV, manage tailings for

safety and with end game of lease relinquishment

uppermost

• Consider constraints over time, including changes in ore

grade and geochemistry

• This will likely bias tailings management away from slurry

disposal, unless enhanced operation and control is in

place

• If upstream construction is favoured, tailings disposal must

be managed to ensure a low rate of rise, discharge away

from dam, and decant pond/stored runoff stored well away

from dam – Avoid loose, liquefiable tailings and tailings

that cannot readily be rehabilitated



Alternative Approaches to Tailings

Disposal, Storage and Rehabilitation

• Recognise high cost of managing and rehabilitating

tailings slurry

• Develop cost-effective dewatering of tailings addressing:

– Perceived high cost of filtration

– Difficulty in scaling-up filtration for high production rates

– Need to ensure stability, which may include compaction

• Develop cost-effective co-disposal of filtered tailings and

scalped waste rock addressing:

– Required moisture content of tailings

– Scalping (crushing, screening) of waste rock

– Mixing of filtered tailings and scalped waste rock

– Transportation of mixture

– Need for compaction and encapsulation of PAF material


2018 tailings and mine waste onference lternative

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